Hi John Hess from Filmmaker IQ.com - in our course video Diving Into Dynamic Range someone
asked what a camera with Dual ISO, like the Blackmagic Pocket Cinema Camera 4K, does with
Dynamic Range.
Well I dug around and noticed that the BMPCC 4K’s dual ISO sensor is actually a great
demonstration of some of the fundamental principles outlined in my Dynamic Range Video.
If you haven’t watched it yet, you should, you really should along with our deep dive
into the Science of Exposure - both of those videos provide the background for what we’re
going to discuss about today.
So let’s get right into it.
First, let’s recall what ISO is.
ISO is not the quantum sensitivity of the sensor itself - the sensor sensitivity is
set at the point of manufacturing.
A certain number of photons hit the silicon sensor and produce a certain number of microvolts
of electricity - I get into the mechanics of this on my Science of Digital Sensors video..
But from this one single sensitivity, we send those microvolts into some microcircuits that
amplify and create an analog signal which is then sent into Analog Digital converter
and processed.
We can change the sensitivity of the entire imaging system by changing how much gain we
apply to the signal both before and after the ADC.
ISO is a standard rating for film speed.
At any given ISOrating a certain predetermined amount of light should return middle gray
regardless of the camera brand - though there is a lot of wiggle room but that’s another
topic all together.
So what’s with this concept of “Dual ISO”?
Well a sensor that has been given a lot of light presents noise differently than a sensor
given very little light.
Some system designs incorporate alternative strategies for noise cancellation when the
camera is engaged in High ISOs as opposed to Low ISO settings.
Let’s diagram this out: We start out with the same photosensitive silicon slab which,
as I said before, does not change quantum sensitivity.
When a lower ISO is engaged, the signal is sent through a basic amplifier which applies
an analog gain to the signal before it is sent to the Analog Digital Converter to convert
to digital.
Then a digital gain is applied and the signal is recorded onto a codec.
When a high ISO is engaged, the signal coming off the sensor instead takes another path
through a more powerful amplifier which has a higher gain and additional noise cancellation
circuits before going to the ADC where again more digital gain is applied and the signal
is recorded to media.
So what that all boils down in practice is we essentially get a camera that behaves in
two different cameras depending on our ISO setting.
As of this video I believe Panasonic, Sony, RED and now Blackmagic are offering cameras
with this Dual ISO capability.
I may have missed a few but right now I’m just going to focus on nnthe 2018 Blackmagic
Pocket Cinema Camera 4K as an example of how this feature will affect your approach to
shooting.
This is the official dynamic range chart from Blackmagic for the BMPCC4K.
Immediately you should identify three regions.
According to Blackmagic literature, the block on the left has a "native ISO of 400" and
the area on the right has a "native ISO of 3200".
Now I'm not a big fan of the term "Native ISO" because I think it hides some strategic
benefits of ISO but I digress, you'll see more what I mean as we continue on.
Notice the area ISO 100-1000 is one solid block rectangle - there is no change in 13.1
stops of dynamic range here.
The camera just slides what is middle gray up and down - this is done by adjusting gain
applied after the ADC - or digital gain.
On stills cameras this might be called an ISO invariant range.
One caveat with the Blackmagic Cinema Pocket camera - if you record RAW this digital gain
is recorded only as metadata - if you were to start recording and adjust the ISO inside
this range mid-recording, you would not record those ISO changes.
But if you shot in ProRes or DNxHR you would actually see the image brightness shift.
The next rectangular area is between ISO1250 and 6400 this time with 12.3 stops of dynamic
range.
This represents the new path and the addition analog gain applied.
Then from ISO 8000 you see this stair stepping down and we get decreasing dynamic range - I
believe at this point we’re just cranking up analog gain.
Let’s see how this plays out in real life.
I rented a Blackmagic Pocket Cinema Camera 4K for a few days and ran a bunch of tests.
Shooting my dynamic range tester - and yes it has gotten a little out of whack since
I last used it - but let’s focus on the brightest light on the right.
At ISO 100 shooting in LOG, the brightest I can expose this to is around IRE 80 - if
we consider middle gray to be around IRE 50 - that’s gives us 2 stops to work with above
middle gray - just as the official Blackmagic dynamic range chart shows.
Keep in mind - I cannot expose anything higher than IRE 80 - the camera literally clips at
IRE 80.
Now watch as I step up the ISO - notice on the vectorscope everything just shifts upwards
even that right spot light - and the clipping point or max exposure ceiling.
So by the time I’m hitting ISO 1000 - I now have 5 spots of light above IRE 50 - again,
my tester needs a tuneup but you get the idea - 5 stops of light match up pretty closely
to 5.3 on the official Blackmagic chart.
But now watch what happens when we make the leap from 1000 to 1250.
It’s not a shift up - The top two spots are now hitting the ceiling with the third
pretty close to clipping - again this was illustrated with the dynamic range chart.
Making the leap from 1000 to 1250 immediately chops off about 3 stops from the top end.
But then look at the bottom - check out that big fat noise line from ISO 1000 and compare
that to the much tighter noise floor of ISO 1250 - we gain about 2.5 stops in the darks.
Now I’m showing you LOG footage because that’s the way the camera can visually squeeze
13 stops into what you’re seeing now as 8 bit video.
This will also apply if you’re shooting RAW but as I said the ISO only recorded as
metadata except for when you make this jump between the two different ranges of the Dual
ISO.
If you’re not shooting LOG or RAW then you’re not getting the full dynamic range for the
camera so the rest of what I say may or may not have mileage.
Technical tests out of the way let’s talk about how you best approach shooting with
Dual ISO.
The trick is to know the kind of image you are ultimately trying to produce.
For a good way to visualize this - let’s bring in Ansel Adams’ Zone System we talk
a bit more on the Zone System in my Exposure video.
Zone V is middle gray.
Zones VI-X are above middle gray and Zones 0-IV below middle gray - each zone is one
stop higher or lower than the surrounding ones.
If your scene is largely in the shadows - that means your shot is mostly below Zone V - Zones
I-III - dark objects with lots of textures in shadow: You want to shoot as low an ISO
as you can to give yourself as much room as possible for data below middle gray.
I’m not talking underexposing here - you want to expose middle gray for what middle
gray is - but open up your exposure as much as you can that’s shutter speed and aperture,
if you’re on a controlled set - add more light, then shoot at the lowest ISO you can.
If we shot ISO 100 for example we would have 11.1 stops below middle gray.
However the Dual ISO, gives us a second useful option.
Take a look at this shot which I exposed at f2.8, ISO 400 1/24 shutter - unfortunately
there’s not enough light to use any lower ISO here.
At ISO 400 I’ve got 9.1 stops below middle gray before we get lost in noise.
Compared to ISO 1000 which only has 7.8 stops below middle gray - you can easily see the
noise starting to sneak in.
But now let’s make make that Dual ISO leap to ISO 1250 and immediately you see the noise
is much less noticeable - I went from 9.1 stops below middle gray at ISO 400 to 7.8
stops below middel gray at ISO 1000 and then 10 stops below middle gray using ISO 1250.
That jump from one part of the dual ISO to the other, added more shadow detail even with
a lesser exposure.
Here’s another dark scene example: this time I was able to expose ISO 100 at f2.8
and shutter speed of 1/24th a second.
Shooting at ISO1000 using f2.8 and shutter of 1/200 - roughly the same exposure but the
higher ISO obviously results in more noise in the shadows.
Then if we take one step up from ISO1000 to ISO1250 shooting same f-stop but 1/250th - the
noise floor plunges away.
It’s does literally feel like we're shooting with a brand new camera.
The rule applies - In a dark scene shoot the lowest ISO possible.
But with dual ISO you’ve got that second much higher baseline to start from.
But even if you have a well lit set, reserve the lower ISOs, the ones starting at 100 and
1250 for scenes that have a lot of shadow in them.
The other side of the rule - in scenes where there is a lot of light and the most important
things are going to be above middle gray - that’s zones VII-X - shoot the highest ISO given
your tolerance for digital noise.
Again, I did not say over expose, expose properly but use the highest ISO.
And of course Dual ISO puts a caveat in this situation as well.
The best demonstration of this is shooting backlit clouds.
Clouds by their very nature are going to be a stop or so above middle gray or Zone VI
or VII - but back lit clouds are going to go much higher.
Here’s a shot of some backlit clouds at ISO 1000 1/1000 shutter speed at f/6.2.
Now compare that with this second shot: ISO 125 1/125 shutter speed at f/6.2.
We dropped down 3 stops in ISO and made up with a 3 stops in shutter speed but all the
detail in our clouds is completely washed away.
You can see this in the Blackmagic chart - With ISO 1000 we have 5.3 stops above middle gray
to work with - but with ISO 125 we’re limited to only 2.3 stops - that’s not a lot.
Now watch what happens when we step up from ISO 1000 to ISO 1250… the same loss of detail
occurs because ISO 1250 also only has 2.3 stops above middle gray.
So in a bright situation focusing on highlights, shoot the highest ISO that gives you tolerable
noise, but when shooting dual ISO make sure you don’t cross over into that next tiered
ISO.
Now you probably won’t be shooting such extremes highlights in everyday normal production
and it’s often perfectly acceptable to let hotspots clip lose detail.
Again let’s look at the Ansel Adams Zone System: 3 stops above middle gray is Zone
VIII - which Adams identifies as textured snow.
With low ISOs on this BMPCC4K textured snow run the risk of being untextured pure white.
But if you aren’t shooting snow - or in my case since I live in Southern California
I have to shoot backlit clouds - then you could potentially get away with clipping the
highs in every day shooting.
Most of the time you will be shooting scenes that are 3 or so stops above and below middle
gray.
Shots like this lake shot contain both highlight and shadow detail but nothing too extreme
in either direction.
Something like this I may prefer to go with as little noise as possible shooting so ISO
100 and live with only 2 stops above middle gray.
After all the only thing that is above middle gray is the blue sky which doesn’t have
much detail anyway.
But if you’re finding yourself clipping when shooting these outdoor shots - say you’re
shooting into the sun, start pushing the ISO higher to give yourself more room in the highlights
at least up until you have to make that switch to the other part of the Dual ISO.
I hope with our exposure and dynamic range videos that I’ve given you a perspective
on how camera exposure works as a strategy.
There is no one size fits all approach - the more you think about how you want the final
result to look or at least the more you think about how the camera can best capture the
data you’re interested in, the more fine tuned you can get your exposure practices
and the better your final image.
But it takes practice, understanding the basic principles and, most importantly, getting
an intimate feel for how your camera works.
The numbers I stated here are only for the Blackmagic Pocket Cinema Camera 4K.
But every camera has it's own personality even if they are very much similar.
Now one last word on Dual ISO.
It’s tempting to look at this strategy and think that all cameras should utilize this
technique.
But Dual ISO isn’t the only strategy out there and it certainly won’t be the last.
Every technique has a tradeoff in terms of cost and engineering.
As neat as dual ISO is in some ways - it can also tangle people up who don't understand
it which is why I felt it was important to put this video together to explain this interesting
quirk in the way the Blackmagic Pocket Cinema Camera handles dynamic rant.
If you liked this video throw us a like and subscribe and consider becoming a patron on
Patreon - every little bit does indeed help.
Thanks to our A list patrons you guys rock.
Don’t forget to check out our merch store - new designs coming very soon I promise.
If you didn’t like this video I can’t think of a better insult than to buy a pallet
of premium cotton Ts for a sacrificial bonfire to your pagan gods who reside in the shadows
and spew digital noise for all of us to capture.
Until next time my friends - go out there and make a great exposure.
I’m John Hess and I’ll see you at Filmmaker IQ.com